Help with a simple 3d Print
You have a lot of big and ugly n-gons and non planar geometry.

Most of it can be fixed by bridging edge loops.

Start by selecting the faces of the n-gons on the main ring and delete them.



Next thing is making it so both sides of this ring have the same number of vertices for each loop.

The center circle has 32 vertices, the outer one only has 30...

How would do you know how many are selected? look at the top of the screen:



In any case you are missing two vertices on the outer ring, so select this long edges, press w to subdivide, and make one subdivision on each.



Now that you have two rings with the same number of vertices you can do Bridge Edge Loops.



The main ring has now some nice quads.



Repeat this for the bottom part of the object.



On the side you have some triangles at the top and bottom



To turn all of this area into quads, select these edge loop:



and do Edge Dissolve to get this:



Repeat this for the other side.

Now on to the most problematic area, delete this faces to get rid of this n-gons on top and bottom of the object :



The next task is to connect the inner ring to the outer shape. The ring has 32 veritces and the outer shape only 8, so we need to create some more vertices.

2 loop cuts here:



11 cuts here:



And 11 more on the other side. Next is selecting the edges on the ring and the outer edge and connecting them with Bridge Edge Loops.



To get something like this:



Repeat this on the bottom and on the other side of the object.

Your object should be now all quads and manifold...

Last stop is making sure your normals are consistent, so select all the faces and press CtrlN